In the run up to the US contests, both Democrat contenders have threatened to pull the US out of the North American Free Trade Agreement (Nafta), which has seen a surge in imports of everything from cars to avocadoes over the 3,000 km Mexico-US border.

Mexican and US commerce ministers responded with a vigorous defence of Nafta, claiming in a meeting in the resort of Los Cabos on Thursday that free trade has brought huge economic growth.

And both Democrat and Republican candidates have also continued to trade blows over illegal immigration in the vast "lone star" state of Texas, which borders four Mexican states and has a swelling Hispanic population.

Mexican politicians have also warned US candidates not to use migrants as a punch-bag to gain votes.

Border defences

"The candidates have to understand that attacking migrant workers will not get them elected"

Jose Jacques Medina, Mexican congressman and migrant activist

Leading Republican candidate John McCain, meanwhile, has fought off attacks that he is soft on illegal immigrants after he co-authored a bill to legalise undocumented migrants in 2005.

In an attempt to win back the conservatives, McCain has focused his Texas campaign on calls for stronger defences on the Mexican border.

But anti-immigrant groups such as the Minutemen have fanned out across the huge state in support of his conservative rival Mike Huckabee, who they say has solid anti-illegal immigrant credentials.

Huckabee, a former Southern Baptist minister, appeared at a border patrol centre in Laredo on Saturday flanked by Minute Man founder Jim Gilchrist and kick-boxing actor Chuck Norris, another outspoken critic of illegal immigration.

More than half of the 12 million undocumented migrants toiling in US kitchens, fields and factories are from Mexico and they sent home almost $24 billion in remittances last year.

Mexican politicians have promised to use their weight in Hispanic communities to support the candidates who offer the best immigration policy.

"The Hispanic vote will be decisive in choosing who will be the next person in the White House," Jose Jacques Medina, a Mexican congressman who was a migrant activist in Los Angeles for more than 30 years, told Al Jazeera.

"The candidates have to understand that attacking migrant workers will not get them elected."

Now the three leading candidates – McCain, Barack Obama, and Hillary Clinton – all favour some kind of reform to legalise more migrant workers.

Obama visited the border line near Brownsville on Friday, saying the immigration issue was a matter of "conscience" before posing with traditional Mexican musicians and eating a taco.

Political promises

Both Obama and Clinton have threatened toopt out of Nafta [AFP]

On the issue of Nafta, both Obama and Clinton have threatened to opt out of the treaty unless it is renegotiated to protect labour and the environment.

Both candidates appear to be using this position to gain the votes of blue collar workers in the state of Ohio, where many factories have outsourced production to Mexico in recent years.

In Texas, however, many farmers and other producers have made huge gains from southbound trade.

However renegotiating the treaty could be a torturous process, while pulling out of Nafta - which is in its 15th year - would be devastating for many US businesses, says Federico Estevez, a Mexican political analyst.

Cross-border trade between Mexico and the US hit $347 billion in 2007.

"Obama and Clinton are making promises they are not going to be able to keep," Estevez said.

"It's just hot air to gain the votes of angry Americans who lost their factory jobs. Nafta is presented as the root of all evil."

'Hurting workers'

Mexican farmers marched in opposition to theNafta treaty in January [Reuters]

However, feelings towards the free trade deal are also mixed in Mexico.

Felipe Calderon, the Mexican president, is a staunch defender of the accord, saying it has helped create millions of jobs.

But many small farmers say they have been devastated by competition from imports of cheap US corn, bean and other crops backed by vast subsidies.

In January, 80,000 people marched through Mexico City with horses, cows and tractors, calling for tariffs to be put back on staple Mexican crops.

"Obama and Clinton are not alienating Mexicans with their calls to re-negotiate Nafta," said political analyst Jose Antonio Crespo.

"Many here feel that even if the accord has helped business, it has hurt many workers in both countries."